Gas turbine engines are known to include a compressor for compressing air, a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. Gas turbine engines using annular combustion systems typically include a plurality of individual burners disposed in a ring about an axial centerline for providing a mixture of fuel and air to an annular combustion chamber disposed upstream of the annular turbine inlet vanes. Other gas turbines use can-annular combustors wherein individual burner cans feed hot combustion gas into respective individual portions of the arc of the turbine inlet vanes. Each can includes a plurality of main burners disposed in a ring around a central pilot burner.
During operation, the combustion process can generate combustion oscillations, also known as combustion dynamics. Combustion oscillations in general are acoustic oscillations which are excited by the combustion itself. The frequency of the combustion oscillations is influenced by an interaction of the combustion flame with the structure surrounding the combustion flame. Since the structure of the combustor surrounding the combustion flame is often complicated, and varies from one combustor to another, and the combustion flame itself may vary over time, it is difficult to predict the frequency at which combustion oscillations occur. As a result, combustion oscillations may be monitored during operation and parameters may be adjusted in order to influence the interaction of the combustion flame with its environment.
In many combustors, a pilot burner is surrounded by other main burners. The pilot burner is used to stabilize the combustion flame resulting from the mix of the fuel and air from the pilot and main burners, because a relatively rich fuel/air pilot flame inside the combustion flame provides stability for the relatively lean fuel/air ratios coming from the main burners. This stability in turn reduces combustion dynamics. Since the pilot flame has an ability to influence the combustion flame, and thus the interaction of the combustion flame with its environment, adjusting the pilot flame has been used as a way to influence/control combustion dynamics.
Some systems employ diffusion pilot burners where fuel is sprayed directly into the combustion flame. A diffusion pilot burner may be used because diffusion burners are very stable. Some burners are premix burners. In a premix burner, a supply of fuel is injected into an airstream within the burner, and is often swirled together via a swirler to mix the fuel and air. The mixed fuel and air is then delivered to the combustion flame. Premix burners are often used as main burners surrounding a pilot burner. The fuel/air ratio from burner to burner may be varied by separately controlling the fuel flow to different burners, which is known as staging. Some pilot burners employ both a diffusion stage and a premix stage to supply fuel and a fuel/air mixture respectively to the combustion flame.
Adjusting the pilot flame of a diffusion burner by adding more fuel may stabilize combustion, but may have adverse effects as well, including increased fuel consumption and poor emissions. As a result, there is room in the art for improvement of control of combustion instabilities in a gas turbine engine.